"I hereby announce that I am resigning as the chairman of the Labor Party and ending my career as an active politician. For me, as a politician, the Labor Party constitutes a major stage of life, the sentiments are mixed as it was and still is the first political organization, which I joined as a member and then became its leader," he said in a letter to fellow party members on Tuesday.
Uspaskich explained that he had decided to resign in order to "provide a new impetus for the party, new ideas, and youthfulness".
Speaking with BNS, Viktoras Fiodorovas, the elder of the Labor Party political group in the Seimas, said Uspaskich's decision was unexpected.
The party's praesidium is scheduled to meet later on Tuesday to approve the interim leader, likely to be one of the seven vice-presidents.
A party convention to pick a new leader is due to be held within a couple of months.
Fiodorovas says Uspaskich will remain an active party member and will represent the party in the European Parliament.
The founder of the Labor Party, Uspaskich has led the party intermittently since 2003. He was last elected to lead the party in 2018 and re-elected in November, 2021.
Having some 10,000 members, the Labor Party has nine seats in the Seimas.